The present invention relates to a semiconductor memory device, and more particularly to a voltage pumping device for generating a high voltage that is a boosted voltage.
In general, a DRAM is a random access memory in which data can be read/written from/into a memory cell consisting of one transistor and one capacitor. In the DRAM, an n-channel metal oxide semiconductor (NMOS) transistor is used as the transistor constituting the memory cell. In order to turn on this NMOS transistor to read or write data from or into the cell, it is necessary to apply a boosted voltage of VDD+Vt or higher to the gate of the NMOS transistor.
Therefore, in order to drive word lines of the DRAM device, there is a need for a voltage pumping device that generates a high voltage VPP, which is the boosted voltage.
FIG. 1 is a block diagram showing the configuration of a conventional voltage pumping device.
As shown in FIG. 1, the voltage pumping device includes an oscillator 100 for generating a pulse signal VPOSC in response to a high voltage pumping enable signal Enable, and a high-voltage pump 200 for pumping a high-voltage VPP in response to the pulse signal VPOSC from the oscillator 100. With this configuration, the voltage pumping device generates the high voltage VPP.
On the other hand, the high voltage VPP is frequently used in an auto-refresh operation, among a plurality of operations of the DRAM. Particularly, the high voltage VPP is more frequently used when a low supply voltage VDD is applied or when an auto-refresh period tRFC is short.
FIG. 2 illustrates a practical drop in the high voltage VPP resulting from the use of a large amount of current when the low supply voltage VDD is applied or when the auto-refresh period tRFC is short. The supply voltage VDD may be dropped due to the frequent use of an internal voltage Vint, thereby adversely affecting other peripheral circuits using the supply voltage VDD.
That is, the drop of the supply voltage VDD results in a level reduction in the internal voltage Vint which is used as a drive voltage of the oscillator 100 generating a pumping period of the high voltage pump 200. For this reason, the period of a pulse outputted from the oscillator 100 becomes longer, thus degrading a high voltage pumping capability.
Consequently, a high voltage driving capability is degraded, resulting in a faulty operation of the DRAM.